Process of joining bodies with boron nitride



Feb. 6, 1968 A.BAER ETAL 3,367,811

PROCESS OF JOINING BODIES WITH BORON NITRIDE Original Filed Nov. 8, 1962CARBON United States Patent 3,367,811 PROCESS (9F .lOlNlNG BODHES WETHHURON NlTRlDlE Charles A. Baer, Princeton, N.J., and Philip J. Clough,Reading, and Robert W. Steeves, Nahant, Mass assignors, by mesneassignments, to National Research Corporation, Cambridge, Mass, acorporation of Massachusetts Original application Nov. 8, 1962, Ser. No.236,365, now Patent No. 3,245,674, dated Apr. 12, 1965. Divided and thisapplication Aug. 10, 1964, Ser. No. 392,994

2 Claims. (Cl. 156-89) ABSTRACT OF THE DHSCLUSURE The high temperaturereaction product of boron nitride and aluminum is used to securetogether pieces of carbon or refractory oxides to form a joint which isresistant to high temperature molten aluminum.

This invention relates to coating and more particularly to the coatingof various substrates with aluminum. This application is a division ofthe copending application of Baer et al., Ser. No. 236,365, filed Nov.8, 196-2, and now Patent No. 3,245,674.

In the vacuum evaporation of aluminum where aluminum is heated to anelevated temperature on the order of 1200 to 1300 C., or above, one ofthe principal technical problems to be solved has been to find acrucible which is resistant to attack by the high-temperature aluminum.The same problem is encountered when a portion of thealuminum-evaporating source which is at elevated temperatures is exposedto a high density of aluminum vapors. This situation arises in thosecases where it is desired to deflect or concentrate the flowing aluminumvapors to provide, for example, lateral or downward evaporation.

Some progress has been made in providing aluminumresistant structures,but these structures have not always been adequately simple to constructor cheap to maintain. While some progress has been made, greatdifliculty has been experienced in providing a complete satisfactorymethod for producing such a source, particularly one which is of acomplex geometric shape. Equally, the art has only with difliculty beenable to produce a stream of aluminum vapors which is directed laterallyor downwardly from the source. Such a stream is particularl useful whendiscrete objects such as nuts and bolts and powders are to be coated, orwhen two sides of a continuous substrate are to be coated at the sametime.

Accordingly, it is a principal object of the present invention toprovide a method for producing a source of aluminum vapors which canhave wide latitude of design and still be reasonably simple andinexpensive to construct.

Still another object of the invention is to provide a source of aluminumvapors of the type described above which is simple to manufacture anduse.

Another object of the invention is to provide a source of the above typewhich can be produced from relatively inexpensive materials.

Still another object of the invention is to provide a means forconfining high-temperature vapors to produce a stream of aluminum vaporsmuch more concentrated than can be produced by previously-knowntechniques.

Still another object of the invention is to provide a mechanism forproviding a high intensity stream of aluminum vapors capable ofproviding rapid coating of a discrete object with minimum transfer ofheat to the object.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

3,36 7,8 l l Patented Feb. f5, 1968 The invention accordingly comprisesthe process involving the several steps and the relation and the orderof one or more of such steps with respect to each of the others and theapparatus possessing the features, properties and the relation ofcomponents which are exemplified in the following detailed disclosure,and the scope of the application of which will be indicated in theclaims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the drawing which is a diagrammatic, schematic,sectional View of one preferred embodiment of the invention.

In the present invention, a container for confining molten aluminum atelevated temperatures is provided by furnishing a refractory base 10which is in the form of a crucible or other structure suitable forconfining a substantial pool of molten aluminum. This base 10 isprefably formed of carbon or a refractory oxide such as magnesia,alumina or zirconia, or a refractory silicate such as zircon and thelike. The base preferably does not contain any substantial quantities ofmaterials which will outgas as the base is heated in a vacuum chamber tothe elevated temperature of 1200 C. and above, which is necessary foraluminum evaporation. For convenience, the invention will be initiallydescribed in connection with the utilization of a carbon cruciblewithout attempting thereby to limit the invention.

A carbon crucible, which may be in the form of a cylindrical bucket orlong boat, is formed 'as a solid piece or built up out of smaller piecesof carbon suitably attached together by carbon pins or the like. Thewhole interior surface of the crucible, which is to be exposed to moltenaluminum or aluminum vapors While the surface is at an elevatedtemperature, is then coated with a slurry of boron nitride 16 to athickness on the order of M inch.

This slurry is then dried, such as by baking in an oven, at atemperature on the order of 200 F. to drive off the water or othermedium for forming the liquid phase in the slurry. The thus preparedcrucible is positioned in a vacuum chamber and suitably supported so asto be heated by an induction coil, for example. Such a crucible has anextremely long life and is capable of operation at elevated temperaturesin contact with molten aluminum for many hours.

While the exact reason is not fully understood, it has also been foundthat the addition of titanium, zirconium, hafnium, vanadium, niobium ortantalum to the molten aluminum tends to maintain an appreciably higherevaporation rate. This is believed to be the result of minimizing anyappreciable concentration of aluminum carbide in the melt due to pinhole porosity of the boron nitride coating which would otherwise permitpenetration of the coating and attack of the crucible by the aluminum.

After a period of operation of the crucible, it was found that the boronnitride layer has been converted to an extremely hard, dense compoundwhose identity has not been clearly established. This is believed to bea reaction product between boron nitride and aluminum, and it has beenfound to form in the vapor state as Well as in the liquid state; thatis, by reaction of boron nitride with aluminum vapors as well as thereaction of boron nitride with molten aluminum.

This surface can also be formed, for example, when a cover is desiredover the crucible to provide lateral or downward evaporation from thecrucible. In this case, a deflecting top 14 is provided over thecrucible, a suitable opening 12. being left between one portion of thecover and the main body of the crucible. This cover is also preferablyformed of a refractory material, such as carbon, similar to the materialof the crucible. It is equally coated with a layer of boron nitride anddried. The abutting surfaces of the crucible and cover are coated with aboron nitride layer. The crucible and cover are placed in the vacuumsystem, the crucible being charged with aluminum. The crucible and coverare then heated to about 1000 to 1300" C. and the boron nitride coatingon the cover is reacted with aluminum vapors coming from the pool ofaluminum confined by the source. These aluminum vapors react with theboron nitride coating at the elevated temperature of the cover. After afew minutes of operation, the aluminum vapors have reacted with theboron nitride coating to form a dense, hard surface which appears to besubstantially impervious to and unreactive with aluminum vapors for longperiods of time thereafter. The joint between the cover and the cruciblehas been converted to a hard, dense mass which is tight to liquidaluminum as well as to the aluminum vapors.

As a result of the techniques described above, it is possible to producesources for molten aluminum having a wide range of geometricconfigurations and a control of aluminum vapors which permits upward,sideways or downward direction of the vapors. The above described methodof forming a vapor-tight joint can also be used to form joints whichalso withstand molten aluminum, such as when a large source is to bebuilt from a number of pieces of carbon.

In one preferred embodiment of the invention, a slurry is prepared bymixing 5 5 grams of powdered boron nitride in 121 cc. of water. Thismakes a paste having the con sistency of whipped cream. This paste isthen applied, such as by a brush, to all those surfaces of a carboncrucible which are to be exposed to molten aluminum or aluminum vapors.This coating is preferably ,4 inch thick. The thus coated carbon body isair dried at about 200 F. Several coatings can be applied in sequence.In one preferred embodiment of the invention, a carbon crucible, havingan internal diameter of 4 and a depth of 2 inches, is thus treated withboron nitride paste and air dried. The crucible is placed in a vacuumcoating tank and is charged with 550 grams of aluminum. The crucible isthen brought up to elevated temperatures on the order of 1200 to 1300 C.At the end of 35 minutes the major percentage of the aluminum has beenevaporated. The tank is opened after the crucible is cooled to about 800C. Another 250 grams of the solid aluminum is added to the remainingmolten aluminum in the crucible. grams of titanium are also added to themelt at this time. The chamber is evacuated again and the crucible isbrought up to operating temperature. The aluminum evaporation rate inthe second run is found to be equal to or greater than the aluminumevaporation rate in the first run, the effect of the titanium additionbeing to remove any small amounts of aluminum carbide formed bypenetration of aluminum through pinholes or cracks in the boron nitridecoating.

In another embodiment of the invention a carbon crucible having aninternal diameter of 2 inches and a depth of 2 /2 inches is providedwith a /2 inch hole near the top thereof. This crucible is also providedwith a close fitting carbon cover. The inner surfaces of the crucibleand cover (as well as the mating surfaces of the crucible and cover) arethen coated with boron nitride and heated as above. This provides asource which produces a concentrated stream of aluminum vapors travelinglaterally from the hole near the top. If the crucible is tilted somewhatthe stream of vapors can be directed downwardly as well as laterally.This arrangement is particularly suited for coating discrete objectssuch as nuts and bolts or powders which are most conveniently coatedfrom above while being supported on a vibrating tray or the like.

While several preferred embodiments of the invention have been describedabove, numerous modifications thereof can be employed without departingfrom the spirit of the invention. The slurry of boron nitride can beprepared using numerous vehicles other than water. However, from thestandpoint of cost and freedom from residual material which might outgasin the vacuum system, water is preferred. Equally wettin agents orbinding agents can be added to the boron nitride, but these have beenfound to be unnecessary. For complex structures they can be helpful.

Similarly, carbon is a preferred material from the standpoint ofstructural strength at elevated temperatures and freedom fromdecomposition at elevated temperatures. Other refractory substances canbe employed; for example, refractory oxides such as magnesia, aluminaand zirconia or refractory silicates such as zircon can be protected bythe application of a boron nitride coating. While metals can be given atemporary coating with boron nitride, the high solubility of all metalsin molten aluminum precludes their use since any pinhole or crack in theboron nitride coating causes rapid failure of any of the metals.

While specific forms of apparatus have not been illustrated, theimproved aluminum vapor source can be employed in many types of coatingdevices such as those shown in the following US. patents: 2,622,401,2,643,201, and 2,879,739 and the copending application of Cerych,Clought and Steeves Ser. No. 795,424, filed Feb. 25, 1959, nowabandoned, to mention only a few of its uses.

Since certain changes can be made in the above process and apparatuswithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process of joining bodies formed of a material selected from thegroup consisting of carbon and the refractory oxides and silicates toprovide a joint capable of resisting attack of aluminum at elevatedtemperatures on the order of 1200 C. and above, said process comprisingthe steps of bringing the bodies into juxtaposition, applying a layer ofboron nitride to the juxtaposed surfaces of said bodies, contacting saidboron nitride layer with aluminum metal, heating said aluminum metal toa temperature on the order of 1000 C. to 1300 C. to combine aluminumwith said boron nitride layer to form a hard, solid reaction product ofsaid boron nitride and aluminum which reaction product strongly unitessaid juxtaposed bodies.

2. The process of forming a joint between two pieces of carbon, thejoint being capable of resisting attack of aluminum at elevatedtemperatures on the order of 1200 C. and above, the process comprisingthe steps of coating the surfaces to be joined with boron nitride,heating the surfaces in a non-oxidizing atmosphere to a temperature ofat least 1000 C. and exposing the thus heated joint to aluminum at saidelevated temperature for sulficient time to cause substantial reactionbetween the aluminum, boron nitride and carbon at the joint with theformation of a hard solid reaction product which strongly unites saidtwo pieces of carbon, said reaction product being unreactive withaluminum and being wettable by aluminum.

References Cited UNITED STATES PATENTS 1,605,205 11/1926 Bellamy 15689 X2,839,413 6/1958 Taylor 10655 X 2,877,532 3/1959 Heine 52305 2,937,1015/1960 Nelson et .al. 10655 2,970,061 1/1961 Burnett 10671 2,996,4128/1961 Alexander 117107.1 X

EARL M. BERGERT, Primary Examiner.

H. F. EPST-ElN, Assistant Examiner.

